1. Field of the Invention
The present invention relates to the linearization of nonlinear networks and in particular concerns an adaptive pre/post-distortion circuit for microwave radiofrequency systems, which is independent of the type of modulation.
2. Description of the Prior Art
The nonlinearity phenomena affect several fields of the engineering. An important aspect, which substantially constitutes the subject of the present invention, is that connected to the nonlinearity phenomenon of the amplifiers used in the electronic systems applied to telecommunications and in particular, without excluding others, to the transmission of digital signals.
In modern radio transmission systems, the use of complex modulations, such as the multilevel ones, requires more and more complex transceiver systems. In particular, in modern digital signal transmission, signals with high peak-to-average power ratios must be transmitted. In order to obtain a good quality of transmission by utilizing these types of signals, it is necessary to keep the extent of the processed signal distortion below a preset level.
This fact often forces the designers to use, for instance in the transmission systems, class A power amplifiers with high back-off values, hence with low energy efficiency.
The main known linearization techniques can be classified into four different main groups or methods: a) Feed-forward; b) Feed-back; c) Baseband digital predistortion; and d) FI or RF analog predistortion. Every specific solution may in turn be implemented according to an adaptive or non adaptive technique.
In the adaptive technique, the linearization apparatus reads the output produced by the power amplifier and, depending on the detected values, updates its internal parameters so as to minimize the extent of the produced distortions. It is easy,to understand how, from a conceptual point of view, it is the ideal solution as it adapts, through a suitable algorithm, the linearization function to the real behavior of the system; unfortunately, in practice, the realization is complex and, above all, expensive.
In the non adaptive technique, on the contrary, the linearization device is adjusted once and for all during calibration, so as to ensure an average behavior which is as satisfactory as possible. The system is more simple, but with lower performances, since there is not a feedback that controls the actual status of the distortion at the output of the transmitter.
WO9705694 concerns a predistortion linearizing device for an RF power amplifier with two amplifiers and a delay line with outputs fed to a Wilkinson combiner producing the predistortion signal fed to a power amplifier. However, such a known device is not easily adaptable to all types of systems and is difficult to be made adaptive. Moreover, it does not allow the correction of distortions of any order and type and it is hardly adaptable to systems with modifiable output power (for example, ATCP).
It is, therefore, the main object of the present invention to provide a pre/postdistortion circuit for microwave radio-frequency system which is independent of the type of modulation and it is so structured to be able to operate both in an adaptive and in a non adaptive manner.
A further object of the present invention is to provide a pre/post-distortion circuit which easily adapts itself to any network topology and which is particularly simple to be used in adaptive systems.
Still a further object of the present invention is to provide a pre/post-distortion circuit that corrects distortions of any order and type and that, particularly for the case of amplifiers, corrects distortions due both to possible drivers and to the terminal or to only one of them.
These and further objects are achieved by a pre/post-distortion circuit according to the invention having the features set forth in the independent claim 1. The invention further provides a method of pre/post-distorting a signal, particularly in radio-frequency systems, according to claim 8. Further advantageous characteristics of the present invention are set forth in the dependent claims The basic idea of the present invention consists in producing a clone of the distortion portion of the signal generated by the power amplifier and feeding it with the input signal, with the right level and the correct phase, in order to eliminate distortions from the output signal.
The invention will become clear from the following detailed description, given by way of a mere exemplifying and non limiting example, to be read with reference to the attached illustrative sheets of drawings.